Yahoo Mail users across the world were unable to access their emails for hours after the site crashed.
In the UK, the problem also affected BT, Sky and TalkTalk's email accounts, which are powered by the US brand's service.
Reports of problems began just after 7am on Thursday, as customers took to social media to say they were unable to log-in or read their mail.
According to Downdectector.co.uk, thousands of people were complaining online about a massive outage to Yahoo Mail services, especially in the UK and US.
In a statement hours later, Yahoo Customer Care said: "Most services are back online. This is a technical issue that affected our services.
"We are sorry for the inconvenience and thank you for your patience as we get everything back up and running."
Although Yahoo users began to report that their emails were working again, many said that emails sent during the outage had not arrived in their inbox.
Others were complaining that they were still unable to use the site at all.
Hayley Cameron wrote to Yahoo on Twitter: "My email inbox is now up and running but there seems to have been no emails from during the outage. Will these eventually be retrieved?
"I'm still missing my emails from early this morning," Tony West also tweeted.
In an earlier statement on twitter, they said: "We understand how difficult this must be for you. We are working especially hard to fix this."
"We will try our best to keep you in the loop and will be posting as soon as it is fixed."
TalkTalk and Sky have also tweeted about the matter.
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Twitter users were asking the tech giant what the problem was and how long the site would be down for.
Others said when they tried to log on, they either got the message: "Unknown Host" or "Will be right back.... Thank you for your patience. Our engineers are working to resolve the issue."
Stephen J Sweeney: "Yahoo! Mail! Is! That's! No! Fun!" while Florie Lhuillier said: "Not been able to log into my account for the past couple of hours, what's going on @Yahoo?"
Starfraz Manzoor said: "This is not a good day to be one of those old schoolers who has stayed with Yahoo Mail."
Another Twitter user called Shelly joked about replying to emails, saying: "Yahoo Mail server is down - guess that means I can postpone catching up on my emails and blame Yahoo (I obviously had every intention of responding to email today, obviouslyyy)"
A spokesperson from Verizon Media representing Yahoo said: “We are aware of a technical issue that is impacting our services.
"We’ve been able to get some services back online already and appreciate our customers’ patience as we work to get everything up and running as soon as possible.”